The present invention pertains generally to the field of sequence analyzers for polyphase power systems, and more particularly to sequence analyzers for three-phase power systems which provide an output voltage having a magnitude proportional to the magnitude of the positive or negative symmetrical component of the power system.
Sequence analyzers find many useful applications in electrical power transmission, distribution, monitoring and control systems. In such applications, sequence analyzers are used as sensors for providing certain data concerning the nature of the voltages present in the three-phase power system. These data are then used as inputs to a decision making process, either by a human operator or by an automatic control system.
One type of prior art sequence analyzer gives a qualitative indication of a given power connection as to whether it is positive or negative sequence, but does not give any quantitative measurement of the magnitude of the symmetrical component. Often such devices comprise some type of wave filter and a neon lamp, for example, which lights when the device is connected in positive sequence, but not negative sequence. Such analyzers are useful to line men and electrical equipment installers to provide a field check of the sequence of a proposed connection prior to installation of the equipment. However, the indicating threshold of devices in this class of prior art sequence analyzers is usually rather indeterminate, and it is not possible to determine the magnitude of the components which may be present, but only their sequence. Analyzers in this class of prior art devices are generally incapable of providing an output magnitude proportional to the magnitude of the symmetrical component, as are the sequence analyzers according to the present invention.
A second general classification of prior art sequence analyzers includes a large number of circuits, also called wave filters or sequence segregating networks, which are capable of producing a desired output signal having a magnitude proportional to the magnitude of a symmetrical component. The prior art includes many individual circuits designed to produce this result. Although the exact configuration of these prior art sequence analyzers varies widely, in general they all comprise a plurality of circuit elements connected in a manner to act upon the input power signal to filter out and isolate a voltage or a current which is indicative of the magnitude of the symmetrical component.
This class of sequence analyzers is, of course, necessary in monitoring systems in which control decisions are to be made on the basis of maintaining the positive and negative sequence symmetrical components within predetermined tolerance limits. For example, control by means of positive and negative sequence symmetrical components has proven to be most useful in protecting motors and other loads from possible damage which could result from continued operation of the motor when faults have occurred on the power line. In addition to being able to detect overvoltage and undervoltage conditions, which could also be detected by voltage averaging type circuits, symmetrical component sequence analyzers have the advantage of being able to detect voltage imbalances among the phases. Motor protection circuits may be designed to operate in response to the signals produced by the symmetrical component sequence analyzer to disconnect the load when an out-of-tolerance condition is detected.
One disadvantage of prior art sequence analyzers for this purpose is that in general they include inductors or transformers. The weight, size and cost of the transformers and/or inductors and their associated magnetic cores makes these circuits unsuitable for many applications.
The present invention achieves advantages in reduced weight, size, and cost by providing symmetrical component sequence analyzers which use only resistive and capacitive elements. The elimination of inductors and transformers also leads to improvements in linearity.